Vertical light-emitting diodes (LEDs) including GaN-based material have become increasingly popular as light sources. Typically, epitaxial material layers including GaN are deposited on non-GaN substrates such as sapphire (Al2O3), due to the low-cost availability of high quality sapphire substrates. However, high-power GaN-based LEDs generate considerable thermal energy during use; this thermal energy requires effective dissipation to enable use of large-size LED sources and to extend the LED operating lifetime.
Although sapphire substrates enable high quality epitaxial layer growth, they are not electrically or thermally conductive and must be replaced with a good thermal conductor prior to final LED device packaging.
Various techniques have been proposed for substitution of sapphire with a thermally-conductive substrate. In one technique, laser lift-off is used to decompose an interface region of GaN using an excimer laser, permitting removal of the sapphire growth substrate. The laser lift-off method of removing sapphire substrates is disclosed in U.S. Pat. Nos. 6,455,340, 7,001,824 and 7,015,117. However, the current laser lift-off approach for making GaN light emitting diodes is incompatible with conventional semiconductor processes as it involves the use of expensive laser equipment. Further, laser lift-off can cause damage to the remaining semiconductor layers such as cracking.
In an alternative technique, chemical-mechanical polishing is used to remove the sapphire growth substrate following bonding of a new host substrate to the epitaxial material layer surface. Using chemical mechanical polishing (CMP), significant cost savings can be realized. Moreover, polishing is a milder method resulting in less damage than laser lift-off. However, the surface of the epitaxial material layer may be non-level due to various device processing prior to bonding of the new host substrate. When bonding a new substrate to a non-level surface, bonding gaps can occur, reducing bonding adhesion and/or causing stresses in the resulting device.
Thus, there is a need in the art for improved techniques for bonding new host substrates to epitaxial layers in LED device fabrication.